I just picked an early PS/2 model, this one: https://mthtrains.com/20-3069-1
I clicked on the Upgrade link and got this file.
pr_triplex_011129c_final_3v.mth
I'm not sure what you're doing wrong, but you may have to right click and select "save as" to get it properly.
The PS/3 Steam Upgrade uses the 3V PS/2 files, do NOT load the chain files.